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Saturday, December 17, 2011

Advent Calendar #17: Fermi's paper snippets

Enrico Fermi is famous for his ingenious ways to arrive at quantitive estimates for the solution of complicated physical problems. One of the most legendary examples is his estimate of the energy released by the first atomic bomb. As Fermi himself recalls in My Observations During the Explosion at Trinity on July 16, 194,

About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T.

Emilio Segrè, who witnessed the event together with Fermi, gives a few more details. In his biography Enrico Fermi, Physicist, he writes that Fermi had done the necessary calculations in advance, "having prepared himself a table of numbers, so that he could tell immediately the energy liberated from this crude but simple measurement."

At Los Alamos, Enrico Fermi had the role of an "oracle": Because of his enormous knowledge and competence in all areas of physics, he was consulted for all kinds of physical problems. However, his mastery of physics could be intimidating to other physicists.

Some twenty-five years ago, I met a colleague of mine emerging from the office of Enrico Fermi. He told me that he had been discussing physics with Fermi; and after a moment's pause asked, "Why am I doing physics? I should probably be a grocer".

In an Amazon.com review of Sanjoy Mahajan's "Street Fighting Math I wrote: ". . . the late astrophysicist and education guru Philip Morrison http://en.wikipedia.org/wiki/Philip_Morrison. In a 1963 letter titled "Fermi Questions" to the editor of the "American Journal of Physics" wrote:

"There is a kind of power over the theoretical and experimental studies in which [the prospective physics graduate student] is engaged which is difficult to define, but whose presence is perhaps more important than the knowledge which is more formal and complete. There is one test of such power which is at the same time a remarkably apt method for its development. The method was the common and frequently amusing practice of Enrico Fermi, perhaps the most widely creative physicist of our times. Fermi delighted to think up and at once to discuss and answer questions which drew upon everyday experience, and upon the ability to make rough approximations, inspired guesses, and statistical estimates from very little data. A few samples are indispensable: How much does a watch gain or lose when carried up a mountain? HOW MANY PIANO TUNERS ARE THERE IN THE CITY OF CHICAGO.

""HOW MANY PIANO TUNERS ARE THERE IN THE CITY OF CHICAGO.""This is simple! Take the number of murders in Chicago, subtract the number of gardeners and voila, this is the number of piano tuners. Everybody knows that the gardener "dunit" if there is one available, if not, the tuner will have to do.